Superconducting devices for most applications except a power transmission cable are used in a magnetic field. In a case of a power transmission cable, when cooling is performed around 4 K using liquid helium, cooling cost is enormous. Therefore, a superconductor used for a power transmission cable is limited to a high-temperature oxide superconductor.
The superconductor used for a power transmission cable is a yttrium (Y)-based wire material. The superconductor used for a power transmission cable is mostly a Y-based wire material manufactured by a metalorganic deposition using trifluoroacetates (TFA-MOD) method.
When a superconductor is applied to a coil, the superconductor is exposed to a magnetic field except a special case such as a superconducting current limiter for suppressing an influence of generation of a magnetic field. A characteristic of a superconductor is reduced by a Lorentz force or the like in a magnetic field. Therefore, it is assumed to use a metal-based superconductor at a temperature of 4 K, to use a Bi-based superconductor at a temperature of 15 to 20 K, and to use a Y-based superconductor at a temperature of 30 to 50 K.
When superconduction is applied to a coil, mainly, a metal-based superconductor has been used at 4 K. However, when a superconducting coil is immersed in liquid helium to be used, the amount of helium necessary at an initial stage of immersion is about four times the volume of a container. For example, the amount of helium necessary for immersion in a 400 liter (L) container is 1600 L.
The price of helium is rising, and will further rise in the future. Helium is produced together with natural gas stored in an upper portion of bedrock. Therefore, helium is not produced in a shale gas field positioned at a lower portion of bedrock. It is considered that the amount of helium produced together with natural gas has already exceeded a peak, and it is considered that helium will be further exhausted in the future to raise the price.
The price of helium has risen approximately to 5000 yen per liter even now. For example, if 1600 L is required for immersion in a 400 L container, helium cost of eight million yen is required. Therefore, in recent years, a superconducting system capable of being used by freezer cooling has been expected. It is expected to use a superconductor not at a temperature of 4 K at which cooling cost is large but basically at a temperature of 30 K or higher.
A superconducting coil requires a technique of vacuum heat insulation. Reduction of the degree of vacuum makes cooling maintenance by a freezer difficult, and stops an entire system. Therefore, maintenance of the degree of vacuum is an important problem in a superconduction application system.
Sealing of many metal welded portions is indispensable for maintenance of the degree of vacuum. When sealing is weakened, vacuum heat insulation cannot be maintained, and maintenance such as vacuum drawing is required again. This case increases maintenance cost or lowers reliability of a system.
In general, a metal welded portion is further deteriorated, and a leak possibility thereof is increased when vibration is applied thereto at a low temperature. A metal bond is a bond held by transfer of a free electron. The mobility of a free electron is reduced by cooling to an extremely low temperature, and a metal bond is weakened. Particularly, it is considered that a large damage is given by reception of vibration at 4 K. Therefore, it is desirable to use a Bi-based superconductor at 15 to 20 K or to use a Y-based superconductor at 30 to 50 K for a superconducting coil.
A Bi-based superconductor or a Y-based superconductor is used as a high-temperature metal oxide superconductor, but is used at a low temperature of a liquid nitrogen temperature or lower in a magnetic field. Particularly, the Bi-based superconductor has an additional problem. In the Bi-based superconductor, a minimum silver ratio at a cross section of a wire material is 60%, and cost thereof is high. Oxygen permeation is required during a heat treatment. In order to improve strength, a precious metal such as gold is required, and cost thereof is further increased. However, it is difficult to obtain a sufficient strength. Therefore, it is difficult to use the Bi-based superconductor for a large device in which a hoop stress of several tens tons is applied to a coil.
Because of the above reasons, manufacturing withdrawal of a Bi-based wire material has occurred successively, and a Y-based wire material is mostly used as a superconducting wire used for a superconducting coil.
In general, a superconductor which is not limited to a Y-based wire material can be present with a magnetic flux as long as it is a second type superconductor. A technique for causing a superconducting characteristic even in a magnetic field by fixing a magnetic flux to a part is an artificial pin technique. It is considered that an artificial pin requires a size of about 3 nm at about 30 K although the size of the artificial pin depends on an application temperature.
In formation of an artificial pin in a Y-based superconducting wire, a TFA-MOD method which has dominated a power transmission cable market has not obtained a satisfactory result in a magnetic field application so far. In the present situation, an effective artificial pin cannot be formed, and a coil used in a magnetic field has not been manufactured even by way of trial. In this system, an artificial pin of Dy2O3 or the like is formed. However, the size thereof is as huge as 20 to 30 nm, and it is considered that such an artificial pin does not act as an artificial pin.
An artificial pin having a huge size is not effective in two points. One of the points is that reduction of the number of artificial pins lowers a pinning effect. When a volume of an artificial pin occupied in a wire material is constant in order to maintain the amount of a current, the number of artificial pins of 30 nm is 1/1000 of the number of artificial pins of 3 nm. Therefore, there is a risk that a magnetic flux could not be fixed sufficiently.
The other one of the points is that the too large size of an artificial pin lowers a pinning effect. When the size of an artificial pin is large, many magnetic fluxes enter the artificial pin. A force of an artificial pin to hold a magnetic flux is applied only to an interface between superconduction and non-superconduction. Therefore, when a plurality of magnetic fluxes enters an artificial pin, a stress of the total amount of a Lorentz force is added, and the magnetic flux goes through the interface. Therefore, there is a risk that a magnetic flux could not be fixed sufficiently.
A physical deposition method such as a pulsed laser deposition (PLD) method or a metal organic chemical vapor deposition (MOCVD) method has been developed as a leading magnetic field application in this situation. In the physical deposition method, an artificial pin is introduced easily, and a BaZrO3 (BZO) artificial pin is often introduced. Many efforts have been made in order to control the size of an artificial pin to 3 nm. In recent years, an artificial pin of about 5 nm has been developed.
As for this BZO artificial pin, a quenching burning accident has often occurred in application thereof to a coil, and it seems that there is no successful example. In addition, it is said that the quenching burning accident occurs at a current value lower than a half of a current value capable of causing energization. For example, when a manufacturer says that energization is possible up to 100 A for a superconducting wire which can be energized at 200 A, it is said that the quenching burning accident occurs by energization at 80 A. As an extreme example, there is a report that destabilization occurs at about 25% of a maximum current value.
A cause of this quenching burning accident has not been necessarily clarified. Realization of a superconducting coil which has suppressed a quenching burning accident is expected.
There is also a report of a result in which uniformity of a generated magnetic field is deteriorated not to satisfy a spec even when the case has not caused a quenching burning accident. Realization of a superconducting coil capable of generating a stable magnetic field is also expected.